Retrofitting vessels to deflect radar signals

ABSTRACT

The radar signature of a vessel is reduced by retrofitting it with an array ( 10 ) of elements ( 15 ) fastened to surfaces ( 14   b ) of the vessel. The elements ( 15 ) have planar faces ( 11-12 ) so oriented that incident radar signal ( 20 ) is reflected away from its angle of incidence.

FIELD OF INVENTION

The present invention is directed to improvements in or relating tovehicles and structures, and is more particularly directed to methodsfor reducing the radar signature of structures by directing the radarreflections from structures away from specified directions.

BACKGROUND TO THE INVENTION

Air-borne and water-borne defence vessels are under constant threat ofdetection and possible attack from non-friendly artillery such asground-based, sea-going or air-borne weapons systems. Vessel detectionis usually by means of radar, since conventional vessels, to a greateror lesser extent, all possess a radar signature.

Radar operates by transmitting a pulse of electromagnetic energy andmeasuring the time between the transmitted pulse and the receipt of thereflection of the pulse from a target. The range at which a target canbe detected is a function of, amongst other things, the intensity of thepulse energy and the size of the target (also known as radarcross-section) with respect to the frequency of operation of the radar.In its simplest form, the best return of the electromagnetic pulse tothe radar is generated when the surface reflecting the pulse is normalto the direction from which the pulse is transmitted. In this example,the strength of the return signal is proportional to the square of thearea of the surface and the square of the radar frequency.

A number of proposals have been put forward with a view to reducing theradar signature of such vessels. In general these proposals involvepurpose-building the vessel with radar cross-section reductioncapability. This involves, in the case of a water-borne craft shapingthe hull and other components of the craft. One such design is theso-called “stealthy” vehicle. These vehicles are designed so that theshape reflects radar energy away from the threat sectors into morebenign areas.

For example, an F117 bomber's threat region is forward and below theaircaft. Hence design is such that energy from a radar signal isreflected upwards and sideways from its angled surface facets. Stealthships require protection from sea-skimming missiles which normallyapproach parallel with the sea surface. Such a ship therefore hassloping sides which reflect the radar energy upwards and away from thethreat direction. A sharply defined or specular reflected beam emerges,as opposed to a diffuse reflection.

It will be appreciated that vessels incorporating such designs involvesignificant expense over and above the basic vessel. With tightening ofgovernment spending generally and defence budgets specifically, theprospect of replacing an ageing fleet with a new fleet whichincorporates an expensive design, even if the des may reduce thelikelihood of loss of the vessel when on a war footing, is not anattractive proposition.

For vehicles that are not designed as “stealthy”, the usual method oftreatment to reduce radar cross-section is to cover the vehicle with aradar absorbent material. These materials are usually electricalattenuators such as carbon granules embedded in a membrane that is thenfastened to the vehicle. This material works by attenuating the signalas the pulse energy passes through it towards the reflecting surface,and then again after reflection so that the resulting return signal isreduced. These methods rely on attenuating the radar beam. Radarabsorbent materials of this type are expensive to purchase and install,and create major maintenance problems by trapping moisture and dirt andpromoting corrosion of the substrate such as the ship super structure.Other proposals for radar absorbency have included the use of absorbentrubber sheets and paints.

SUMMARY OF INVENTION

The present invention accordingly provides in one embodiment a methodfor retrofitting a vessel to reduce its radar signature, the methodincluding the step of attaching to surfaces of the vessel structure anarray comprising a plurality of elements, the elements having reflectivesurfaces with substantially planar faces, the arrangement being suchthat when attached to surfaces of the vessel structure the faces areoriented so as to reflect an incident radar signal in a direction awayfrom its direction of incidence for a given range of incidentdirections.

The present invention provides in another separate embodiment an arraywhen used for retrofitting to a vessel to reduce its radar signature,the array capable of being fastened to surfaces of the vessel structureand comprising a plurality of elements, the elements having reflectivesurfaces with substantially planar aces, the arrangement being such thatwhen the array is fastened to the vessel structure the faces areoriented so as to reflect an incident radar signal in a direction awayfrom its direction of incidence for a given range of incidentdirections.

The present invention provides in another separate embodiment a vesselhaving surfaces of its structure retrofitted with an array according tothe invention.

The orientation of the faces is preferably such that the faces areoblique to the direction of incident radar for a given range of incidentdirections. Preferably any edges defining the boundary of the reflectivesurfaces are also oriented so as to be oblique to the direction ofincident radar. In this embodiment of the invention, with a surfaceorientation that is oblique to the direction of incident radar, thestrength of the return signal is then only proportional to the square ofthe length of the edge of the surface normal to the incident radar. Ifthe face is oblique to the direction of incident radar, and the edges ofthe surface are oblique to the direction of incident radar, then onlythe corners of the surface will reflect as point sources. As points haveno spatial dimension, the strength of the return signal from each pointwill vary inversely with the square of the radar frequency. In this waythe radar cross-section of the vessel, and hence its radar signature,can be reduced for a given range of incident radar directions.

The present invention is capable of providing a degree of control overthe direction in which electromagnetic waves are redirected, permittingother directions as well as the incident direction to be avoided whererequired.

A vessel according to the invention is any ship or vehicle requiringdefence against radar threat. A vessel structure according to theinvention includes any surface on the vessel capable of reflecting aradar signal. The structure will therefore, in the case of a water-bornecraft, include the hull, bulkhead, decking, the bridge, any weapons orweapon turrets, and rigging.

In the case of a water-borne craft, incident radar is generally parallelto the sea surface, although naturally in some cases incident radar willemanate from aircraft. In accordance with the invention the reflectivesurfaces are preferably oriented so as to reflect an incident radarsignal by up to about 30 degrees. More preferably the reflectivesurfaces are oriented so as to reflect an incident radar signal by up toabout 15 degrees and more typically by up to about 8 to 10 degrees awayfrom its direction of incidence for a given range of incidentdirections.

A surface to which an array according to the invention is attached willinclude substantially vertical surfaces, substantially horizontalsurfaces, surfaces disposed at an angle to the vertical, and curvedsurfaces.

An element according to the invention may take any suitable form. Theelement will typically be triangular, polyhedral, pyramidal or prismaticin shape or in cross-section. The element may be an elongated triangle,polyhedron or pyramid. The element may be open-sided. It may be a solidfigure. Where an element according to the invention defines an apex, theapex will typically be disposed in a region which is forward (colinearwith respect to the incident radar signal) relative to the planar facesof the reflective surfaces.

An array according to the invention may take any suitable form. It maybe uniform or non-uniform. If uniform, the array may comprise a grid ofuniformly spaced elements having reflective surfaces with substantiallyplanar faces. If non-uniform, the array may comprise a plurality ofrandomly arranged elements having reflective surfaces. The elements maybe uniform or non-uniform in shape or cross-section as required.

The arrangement of elements and/or the manner of installation of thearray on a vessel structure is preferably such that the facets of theelements reflect the incident radar signal away from the threatdirection. The arrangement is also preferably such that the facets ofthe array do not provide to any appreciable extent internal reflectionsources whereby to reduce the likelihood of an incident radar signalbeing reflected towards an adjacent element in the array and in turnreflected back in the direction of the incident radar signal.

The array arrangements may be such that planar faces of the reflectivesurfaces of the elements are not all arranged in parallel planes.

An array according to the invention is preferably formed from alightweight material so as to not substantially increase the overallweight of the vessel. The array may be in the form of a cladding. Theelements may include perforations for the purpose of reducing itsoverall weight. If a perforated element is used, the holes are smallrelative to the wavelength of the incident radar signal to enable theelements to function as a solid reflector. The size and distribution ofthe perforations will therefore be dependent on the wavelength of thedefined threat. The perforations may comprise elements according to theinvention. A typical lightweight and relatively inexpensive materialsuitable for use in retrofitting methods according to the inventioncomprises aluminum. The aluminum may comprise a foil. The elements mayin an alternative embodiment be formed from a mesh, such as a wovenmesh. The mesh will be suitably treated so as to present substantiallyplanar faces. Treatment could include encapsulation within a nonmetallic structure such as a fibre reinforced resin bonded compositematerial or between two sheets of glass where light or vision isrequired. Other materials such as bronze, stainless steel and copper maybe used for forming an array according to the invention and areenvisaged with the scope of the present invention.

The array may be provided in roll form capable of being cut tofacilitate ease of attachment of the any to the vessel. It may beprovided in sheet form. The sheet may include stiffening to enhance itsrigidity or resilience. The sheet may be elongated. In one embodimentthe array comprises a plurality of elongated sheets. The sheets may bejoined together in any suitable manner. In another embodiment the arraycomprises a plate or plurality of plates. The plates may be joinedtogether in any suitable manner.

An element may be formed on or in the array by pressing, impressing,stamping, casting, extrusion or by other suitable means to create thedesired substantially planar ace for a reflective surface of the array.

The element is preferably of relatively small thickness. A typicalelement thickness is in the range of from about 0.25 mm to 15 mm morepreferably in the range of from about 1 mm to 6 mm.

An array according to the invention may be attached to the vesselstructure in any suitable manner. The array may be screwed, welded orotherwise fastened to the structure. If welded, the array may be studwelded to the structure and secured by means of screws. The screws maybe concealed from a given range of incident directions. Other attachmentarrangements are envisaged within the scope of the invention.

The array may be releasably attachable to the vessel. This mayfacilitate ease of maintenance of the vessel structure and/orreplacement or modification of the characteristics of the array.

The present invention provides in another separate embodiment a methodfor retrofitting a building structure to reduce its radar signature, themethod including the step of attaching to surfaces of the buildingstructure an array comprising a plurality of elements, the elementshaving reflective surfaces with substantially planar faces, thearrangement being such that when attached to surfaces of the structurethe faces are oriented so as to reflect an incident radar signal in adirection away from its direction of incidence for a given range ofincident directions.

The present invention provides in another separate embodiment an arraywhen used for retrofitting a building to reduce its radar signature, thearray adapted to be fastened to surfaces of the building and comprisinga plurality of elements, the elements having reflective surfaces withsubstantial planar faces, the arrangement being such that when the arrayis fastened to the building the faces are oriented so as to reflect anincident radar signal in a direction from its direction of incidence fora given range of incident directions.

The present invention provides in another separate embodiment a buildinghaving surfaces retrofitted with an array according to the invention.

A building according to the present invention may include a permanent ortemporary structure.

The present invention provides in one particularly preferred embodimenta method for retrofitting a vessel to reduce its radar signature, themethod including the step of fastening to surfaces of the vesselstructure in elongated sheet form an array comprising a plurality ofuniformly shaped elements being triangular, polyhedral pyramidal orprismatic in shape and having reflective surfaces with substantiallyplanar faces, the orientation of the faces and edges defining theboundary of the reflective surfaces being such that the faces and theedges are oblique to the direction of incident radar for a given rangeof incident directions, the arrangement being such that when fastened tosurfaces of the vessel structure the reflective surfaces are oriented soas to reflect an incident radar signal by up to about 30 degrees awayfrom its direction of incidence for a given range of incidentdirections.

The present invention provides in another particularly preferredembodiment an array when used for retrofitting a vessel to reduce itsradar signature, the array adapted to be fastened to surfaces of thevessel structure and comprising a plurality of uniformly shaped elementsbeing triangular, polyhedral pyramidal or prismatic in shape and havingreflective surfaces with substantially planar faces, the orientation ofthe faces and edges defining the boundary of the reflective surfacesbeing such that the faces and the edges are oblique to the direction ofincident radar for a given range of incident directions, the arrangementbeing such that when the array is fastened to surfaces of the vesselstructure the reflective surfaces are oriented so as to reflect anincident radar signal by up to about 30 degrees away from its directionof incidence for a given range of incident directions.

DESCRIPTION OF PREFERRED EMBODIMENT

The present invention will now be described with reference toparticularly preferred embodiments, in which:

FIG. 1 is a schematic of the reflection pattern of a vessel without anarray according to the invention;

FIG. 2 is a schematic of the reflection profile of the bulkhead of avessel structure to which one element of an array according to theinvention is attached; and

FIG. 3 is a cut-away perspective view of a side of a vessel to which anarray according to the invention has been attached.

Turning to the drawings, FIG. 1 shows a vertical surface 14 acomprising, symbolically, a bulkhead of a vessel structure. In thisarrangement a substantially horizontal incident radar signal (designatedin FIG. 1 as “pulse energy”) string vertical surface 14 a (being normalto the incident radar signal) is reflected from the vertical surface 14a directly back to the radar source (not shown). It will be appreciatedthat a consequence of this arrangement is that the surface by virtue ofits cross-section produces an easily discernible radar signature.

FIG. 2 shows symbolically a bulkhead 14 b to which an array 10 has beenattached. Array 10 comprises a plurality of elements (of which forconvenience only one typical example designated 15 is shown incross-section in this embodiment). The element 15 shown is triangular incross-section and comprises reflective surfaces having substantiallyplanar faces 11, 12 and a leading edge 13. It can be seen from thisembodiment that the plan faces are oriented so as to be oblique to thehorizontal direction of the incident radar (also designated in FIG. 2 as“pulse energy”). Although not clearly shown in FIG. 2, edge 13 can alsobe oriented so as to be oblique to the horizontal direction of theincident radar.

FIG. 3 shows part of an array 10 having a pair of elements 15 attachedto the vertical surface of a bulkhead in the form of a ship's side 14 bby concealed fastenings (that is, concealed from a given range ofincident directions) in the form of screws 17.

Accordingly an incident radar signal 20 in a horizontal direction willbe reflected in a direction 21 away from its direction of incidence onstriking a planar face 11 or 12 of element 15. The angle of incidence ofa reflected signal striking another part of the vessel structure, or thesea or ship deck 16 in the case of a water-borne craft willcorrespondingly be the same. The angle of reflection of the incidentradar signal is preferably up to about 30 degrees away from thedirection of the incident radar signal as shown in FIG. 2. By contrast,the untreated upper vertical surface of the ship's side 14 b willreflect incident radar signal 20 a directly back in the direction of itsdirection of incidence as that surface is normal to the direction of theincident radar signal 20 a.

It will accordingly be appreciated that an array arrangement comprisinga plurality of elements 15 is such that when attached to surfaces of thevessel structure the reflective surfaces 11, 12 of the elements 15 areoriented so as to reflect an incident radar signal in a direction awayfrom its direction of incidence for a given range of incidentdirections. There is also a degree of control over the direction inwhich the electromagnetic waves are redirected, permitting otherdirections as well as the incident direction to be avoided, whererequired. The array 10 is formed from substantially non-porous material.If a reflective mesh with holes is used, the mesh holes are smallrelative to the wavelength of the incident radar signal 20 a to enablethe elements 15 to function as a solid reflector.

In use, and with reference to FIGS. 2 and 3, an array 10 in elongatedaluminum sheet form comprising a plurality of shaped elements 15 oftriangular cross-section and having reflective surfaces withsubstantially planar faces 11, 12 stamped, embossed, cast or otherwiseformed thereon or therein is fastened to a vessel structure by studwelding following by securing with concealed fastenings in the form ofscrews 17. The orientation of the reflective surfaces and, if required,the edges 13 defining the boundary of the substantially planar faces 11,12 is such that the reflective surfaces and the edges are oblique to thedirection of incident radar for an incident horizontal directions, thearrangements being such that when fastened to surfaces of the vesselstructure the reflective surfaces are orientated so as to reflect anincident radar signal by up to about 30 degrees away from the horizontal

The present invention accordingly provides a cost-effective method forreducing the radar signature of an existing vessel and avoids the needto replace an existing vessel with an expensive purpose-built vessel.

The word ‘comprising’ and forms of the word ‘comprising’ as used in thedescription and in the claims does not limit the invention claimed toexclude any variants or additions.

Whilst it has been convenient to describe the present invention inrelation to particularly preferred embodiments, it is to be appreciatedthat other constructions and arrangements are considered as fallingwithin the scope of the invention. Various modifications, alterations,variations and/or additions to the constructions and arrangementsdescribed herein are also envisaged as falling within the scope of thepresent invention.

What is claimed is:
 1. An array for retrofitting to a vessel to reduceits radar signature, the array capable of being fastened to surfaces ofthe vessel structure and comprising a plurality of elements havingreflective surfaces with substantially planar faces, wherein when thearray is fastened to the vessel structure the faces are oriented so asto coherently reflect an incident radar signal in a direction away fromits direction of incidence for a given range of incident directions. 2.An array according to claim 1 wherein the orientation of the faces issuch that the faces are arranged so as to be oblique to the direction ofincident radar for a given range of incident direction.
 3. An arrayaccording to claim 1 wherein any edges defining the boundary of thereflective surfaces are oriented so as to be oblique to the direction ofincident radar for a given range of incident directions.
 4. An arrayaccording to claim 1 wherein the reflective surfaces are oriented so asto coherently reflect an incident radar signal by up to about 30degrees.
 5. An array according to claim 1 wherein the reflectivesurfaces are oriented so as to reflect an incident radar signal by up toabout 15 degrees.
 6. An array according to claim 1 wherein thereflective surfaces are oriented so as to coherently reflect an incidentradar signal by up to about 8 to 10 degrees.
 7. An array according toclaim 1 wherein the elements are triangular, polyhedral, pyramidal orprismatic in shape or in cross-section.
 8. An array according to claim 1wherein the elements are an elongated triangle, elongated polyhedron orelongated pyramid.
 9. An array according to claim 1 wherein the elementsare an apex disposed in a region which is forward relative to the planarfaces of the reflective surfaces and collinear with respect to theincident radar signal.
 10. An array according to claim 1 wherein thearray comprises a plurality of uniform elements.
 11. An array accordingto claim 10 comprising a grid of uniformly spaced elements havingreflective surfaces with substantially planar faces.
 12. An arrayaccording to claim 1 formed from a lightweight material.
 13. An arrayaccording to claim 1 wherein the elements are perforated.
 14. An arrayaccording to claim 1 wherein the array is formed from a mesh material.15. An array according to claim 1, wherein said array is in roll orsheet form.
 16. An array according to claim 15 wherein said sheetincludes stiffening means to enhance the rigidity or resilience of saidsheet.
 17. An array according to claim 1 wherein the elements have athickness in the range of from about 0.25 mm to 15 mm.
 18. An arrayaccording to claim 17 wherein the element is of a thickness in the rangeof from about 1 mm to 6 mm.
 19. A vessel have a structure to which isattached at least one array as claimed in claim
 1. 20. A method ofretrofitting an array to a vessel to reduce its radar signature, thearray being as claimed in claim 1, the method including the step ofattaching to surfaces of the vessel structure the array wherein thearrangement when attached to surfaces of the vessel structure results inthe faces being oriented so as to coherently reflect an incident radarsignal in a direction away from its direction of incidence for a givenrange of incident directions.
 21. A method according to claim 20 whereinthe array is fastened to the vessel structure by welding or by screwfastening to the structure.
 22. A method according to claim 20, whereinthe vessel structure comprises any surface on the structure capable ofreflecting a radar signal.
 23. A method according to claim 20, whereinthe arrangement of elements on the vessel structure is such that thefacets of the elements reflect the incident radar signal away from athreat direction.
 24. A method according to claim 20, wherein the arraycomprises a plurality of sheets capable of being joined together.
 25. Amethod for retrofitting an array to a vessel to reduce its radarsignature, the array being as claimed in claim 1, the method includingthe step of fastening to surfaces of the vessel structure in sheet formone or more arrays comprising a plurality of uniformly shaped elementsbeing triangular, polyhedral, pyramidal or prismatic in shape and havingedges defining the boundary of the reflective surfaces being such thatthe faces and the edges are oblique to the direction of incident radarfor a given range of incident directions, the arrangement being suchthat when fastened to surfaces of the vessel structure the reflectivesurfaces are oriented so as to coherently reflect an incident radarsignal by up to about 30 degrees away from its direction of incidencefor a given range of incident directions.
 26. A retrofitted vessel madeby the method of claim 20.